1. Field of the Invention
The present invention relates to reducing aberration in optical systems. More particularly, the present invention relates to apparatus and methods for reducing polarization aberrations in optical systems such as lithographic imaging systems comprising cubic crystalline optical elements having intrinsic birefringence.
2. Description of the Related Art
In order to increase levels of device integration for integrated circuit and other semiconductor components, device features having smaller and smaller dimensions are desired. In today's rapidly advancing semiconductor manufacturing industry, the drive is to produce such reduced device features in a reliable and repeatable manner.
Optical lithography systems are commonly used to form images of device patterns upon semiconductor substrates in the fabrication process. The resolving power of such systems is proportional to the exposure wavelength; therefore, it is advantageous to use exposure wavelengths that are as short as possible. For sub-micron lithography, deep ultraviolet light having a wavelength of 248 nanometers or shorter is commonly used. Wavelengths of interest include 193 and 157 nanometers.
At ultraviolet or deep ultraviolet wavelengths, the choice of materials used to form the lenses, windows, and other optical elements of the lithography system is significant. Such optical elements preferably are substantially optically transmissive at short wavelengths used in these lithography systems.
Calcium fluoride and other cubic crystalline materials such as barium fluoride, lithium fluoride, and strontium fluoride, represent some of the materials being developed for use as optical elements for 157 nanometer lithography, for example. These single crystal fluoride materials have a desirably high transmittance compared to ordinary optical glass and can be produced with good homogeneity.
Accordingly, such cubic crystalline materials are useful as optical elements in short wavelength optical systems including but not limited to wafer steppers and other projection printers used to produce small features on substrates such as semiconductor wafers and other substrates used in the semiconductor manufacturing industry. In particular, calcium fluoride finds particular advantage in that it is an easily obtained cubic crystalline material and large high purity single crystals can be grown. These crystals, however, are expensive, and certain orientations, such as the <100> and <110> crytallographic orientations are more expensive than others, like the <111> crystal orientation.
A primary concern regarding the use of cubic crystalline materials for optical elements in deep ultraviolet lithography systems is anisotropy of refractive index inherent in cubic crystalline materials; this effect is referred to as “intrinsic birefringence.” For light propagating through a birefringent material, the refractive index varies as a function of polarization and orientation of the material with respect to the propagation direction and the polarization. Accordingly, different polarization components propagate at different phase velocities and undergo different phase shifts upon passing through an optical element comprising birefringent material.
When used for construction of elements of an optical system, the birefringent properties of these cubic crystalline materials may produce wavefront aberrations that significantly degrade image resolution and introduce field distortion. These aberrations are particularly challenging for optical instruments employed in photolithography in today's semiconductor manufacturing industry where high resolution and tight overlay requirements are demanded by an emphasis on increased levels of integration and reduced feature sizes.
It has been recently reported [J. Burnett, Z. H. Levine, and E. Shipley, “Intrinsic Birefringence in 157 nm materials,” Proc. 2nd Intl. Symp. on 157 nm Lithography, Austin, Intl. SEMATECH, ed. R. Harbison, 2001] that cubic crystalline materials such as calcium fluoride, exhibit intrinsic birefringence that scales as the inverse of the square of the wavelength of light used in the optical system. The magnitude of this birefringence becomes especially significant when the optical wavelength is decreased below 250 nanometers and particularly as it approaches 100 nanometers. Of particular interest is the effect of intrinsic birefringence at the wavelength of 157 nanometers (nm), the wavelength of light produced by an F2 excimer laser, which is favored in the semiconductor manufacturing industry. Strong intrinsic birefringence at this wavelength has the unfortunate effect of producing wavefront aberrations that can significantly degrade image resolution and introduce distortion of the image field, particularly for sub-micron projection lithography in semiconductor manufacturing.
Thus, there is a need to reduce these wavefront aberrations caused by intrinsic birefringence, which can degrade image resolution and cause image field distortion. Such correction is particularly desirable in projection lithography systems comprising cubic crystalline optical elements using light having wavelengths in the deep ultraviolet range.